The major objective is to assess the relative influences of the stimulus- and response-reinforcer contigencies on the performance of rats in a free-operant discrimination (Multiple schedule) situation when the discriminative stimuli are located on the operanda. The Multiple schedules will be programmed by alternating the presentations of two retractable levers, each associated with its own reinforcement schedule. The first experiment will study lever-press acquisition in four groups. In the Omission group, omission training, in which a reinforcer will be delivered once every 15 seconds but each response will delay reinforcement for 15 seconds, will hold in the presence of one lever, and extinction in the presence of the other. Subjects in the Yoked group will receive the same pattern of reinforcement and lever presentations as pair-mates in the Omission group. The Differential group will get a reinforcer every 15 seconds, regardless of responding, during one lever's presnce, and extinction during the other's. After initial training, these groups will receive further sessions in which the above conditions will continue except that reinforcers will be delivered in the presence of the levers previously associated with extinction. A fourth group will receive this nondifferential procedure throughout training. The second experiment will study maintenance of an established response. First, lever pessing will be reinforced on a Multiple Fixed Interval Extinction schedule. Then two separate groups will receive the same treatments as described above for the Omission and Yoked groups. Finally, all subjects will get the nondifferential treatment described above.